It was heretofore necessary in such belt manufacture to apply tension to elastic yarns, twist and mix the yarns with other fibers, while maintaining the elastic yarns in the elongated state, and twist and mix the mixed yarns, as shown in FIG. 1, so as to obtain a woven fabric having high elongation as a result of the crimp.
However, the elongation of the elastic yarn has the undesirable tendency to include more than approximately 20 times of the other fibrous yarn. When the yarn is twisted in the conventional manner, the elastic yarn is wound on the periphery of the fibrous yarn to produce an elastic cover yarn, as shown in FIG. 3. Alternatively, fibrous yarn is wound on the periphery of the elastic yarn, as shown in FIG. 4 to produce an elastic yarn. The elastic cover yarn has the disadvantages of lack of elongation and elasticity, while the elastic core yarn has the disadvantages of low elongation and lack of wear resistance. Another conventional prior art twisted yarn comprises mixing the yarns by a twisting method causing them to be partly formed in a wavy or loop shape to thereby produce a ring yarn or a loop yarn. This method, however, causes variation in the thickness of the fabric made of these yarns. When this fabric is provided on the surface of the toothed belt part, the pitch line difference, PLD, (the spacing between the center line of the tensile yarn and the root of the valley between the tooth) is irregular. Such ring yarn has a g/d characteristic of 0.1 to 0.2 g/d according to the rigidity and twisting factor of the fiber in the case of ordinary twisting, and 0.5 in case of strongly twisted yarn, and tends to become such an irregular ring yarn as a result of variable tension applied in the forming of the twisted yarn.